Burnt dusty smell from Transit heater.

My Chausson which I bought is 7 tears old and they never went very far except for the summer holiday to France,, He worked for P&O so got a free trip every year. These were taken in the summer so they never used the van heater. When I got it I have used it more in the few months than they did in a year, but It is a bit chilly now so the heater was put on and the smell of hot dust is quite pronounced and unpleasant. I have tried putting my airline down the vents and putting the blower on full power when I am not in it, even vacuuming with my industrial cleaner but the smell persists. I suppose it will go but I do not like it at all ( It stinks and I sneeze)
Does anyone have a good idea to get the 7 years worth of accumulated dust and dirt out of it WITHOUT taking the dash and heater unit out. It is probably on the actual heater matrix buried depp in the unit.

It will take a long time to burn off the accumulated dust as there is no electric element to burn it off quickly.....But it will go eventually.
As said, pollen filters can become contaminated and they do stink so change it if there is one

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To the OP, you could try getting the engine hot then leaving it running with the heater on full hot fan setting one, this might just burn off the accumulated dust. If you put the fan on higher than pos 1 the dust won't get hot enough to be burned off. After half an hour or so put the blower on full to remove the dust (putting your vacuum cleaner into the heater outlet at this point may help.

Face mask dust filter with cartridges I think for me until it goes away the living area heater is fine I blew that out with the airline. I want to use it in the cold time but only have so many thick jumpers.